The present invention relates to a microwave antenna for wireless interconnection of automation devices, and in particular for networking of remote sensors, actuators and a central control unit in an automated installation.
For many years, there has been an increasing desire to automate the process procedures for the industrial production of products. This is leading to increasing networking of devices and components which are involved in the production processes. Typically, these are sensors for detection of installation or process states, actuators which cause a change in the installation or process states, and control units for producing control signals by means of which the actuators are controlled as a function of the sensor signals. In small installations, the sensors and actuators can be connected directly to the control unit. In larger and extended installations, which require a large number of sensors and actuators, communication networks have already been in use for many years, in order to network the sensors, actuators and control units to one another. A typical example of such communication networks is so-called fieldbuses. These are communication networks which are matched to the specific requirements for applications such as these, in particular with respect to the severe environmental conditions and the typical communication requirement between control units and remote sensors and actuators. Known fieldbuses are the so-called Profibus, the so-called Interbus and the so-called CAN bus. These fieldbuses typically use electrical and/or optical lines for networking the connected devices.
For some years, efforts have also been made to implement the networking of automation devices on the basis of the known Ethernet Standard, which has come into use for the networking of personal computers in home and office applications. In this context, there are also efforts to implement the connection between the devices wirelessly as is already frequently the case with the aid of WLAN in home and office networks. However, the home and office network technology cannot be transferred directly to industrial production environment applications, because the communication requirement and the environmental conditions are different. Typically, there are a large number of metallic objects and moving objects in factory workshops, which can severely influence the propagation of radio waves. On the other hand, the communication between the control units and the sensors and actuators frequently has to take place at very short, cyclically recurring, time intervals, in order to allow a continuous production process, without any disturbances. Furthermore, the requirements for the reliability of the communication link are more stringent when safety-relevant data is intended to be transmitted, on which the operating safety of an automated installation depends. For example, many production installations carry out dangerous movements which must be stopped immediately when an operator approaches the installation. In a situation such as this, the signal from a light barrier which detects the person must be transmitted quickly to the central control unit, and the switch-off command must reach the correct drive in the installation within a defined and guaranteed time period. This is typically in the region of fractions of seconds, in contrast to home and office networks.
There are a plurality of antenna types and shapes for various applications in the field of automation technology as well as in the fields of satellite communication, mobile radio, position-finding, etc. One specific example suggested in amateur radio is what is called an “eggbeater antenna”. This antenna has two circular conductor loops whose ends are connected via a bypass line, with the length of the bypass line corresponding to approximately one quarter of the wavelength of the radio signals to be received. The two conductor loops are connected to a common feed point, at which the antenna signals can be input or output. The antenna is supposed to have a good omnidirectional radiation characteristic. However, it is not suitable for the severe environmental conditions in an industrial production installation.